Conventional card printing devices, such as many time recorders or “punch clocks”, employ an inexpensive direct current motor as a means for drawing in a time card and pulling it to the correct position for printing in a printing column, for example.
In the case of a time recorder, when a time card is inserted into a card insertion inlet, the card is detected by a sensor, triggering the motor that draws the card in, relative to a printing means, to a position that corresponds with the current date. The card is then stopped and printed by a printing head.
Precise control of the card stopping position is critical to ensure that the card is properly printed. In the prior art this has been accomplished by braking by applying voltage in the reverse direction to the motor when the card is a predetermined distance away from the target position thereby rapidly decelerating the forward movement of the card, bringing it, theoretically, to a stop at the target position.
However, the control method employed by the prior art has several shortcomings. When the electrical current applied to the card drive motor is abruptly reversed, enormous stresses exerted on the motor deteriorate its durability. Furthermore, calculation of a precise stopping distance of the card is inherently inexact, particularly at high card speeds because it is difficult to predict the stopping position of the direct current motor used to move the card. This is a problem shared in common by any system employing a direct current motor requiring an accurate stop position from a high operating speed.
Therefore, in order to resolve some of these problems, applicant has proposed in Japanese Patent Application No. 9-316824 stop control which is accomplished through an intermediate reduction in the speed of the card drive motor. The card drive motor is initially operated at a high speed until the card is relatively close to the target position, and then switched to a lower speed allowing the card to decelerate to a lower speed, from which it is easier to stop accurately. The rotational speed of a DC motor used as a card drive motor is detected while drawing in a card at a high speed. The time required to stop the card is calculated from that speed, and the timing of the switch from high to low speed drive is adjusted accordingly.
In practice however, this approach is imperfect. In order to reduce the time it takes to feed a card, the card drive motor must operate at a high speed for as long as possible during the feeding of the card. As shown in FIG. 5, extending high speed drive until just prior to the point at which the card must be stopped does not leave sufficient time for the card to decelerate on its own to a stabilized velocity equal to that of the low speed drive. This means that the card must be abruptly stopped from some speed higher than that defined by the low speed drive, causing enormous stresses to the motor during braking causing deterioration of the motor and reducing the accuracy of the calculated stop position.
Furthermore, as shown in FIG. 6, switching to low speed drive sufficiently early in the feeding of the card to allow it to be stabilized at equilibrium with the low speed drive increases card feed time, reducing performance. These problems arise not only for motors for time recorders but for any direct current motor requiring an accurate stop position in a short period of time.
Motor control presents a similar problem when applied to a printing apparatus such as that found in a time recorder. Prior art impact printing systems have used stepping motors, making print head feed control easy to carry out, but relatively expensive. Direct current motors, have also been used, subject to the limitation that the scanning speed of the print head is likely to vary, affecting print quality. For example, if the drive is running on a partially discharged battery, scanning speed may fall. Attempts to compensate for this by speeding up the direct current motor are likely to result in transient overspeed, also degrading print quality as described above. Employing a variable voltage driver circuit to stabilize scanning is another expensive solution. An impact print head found in existing time recorders or “dot-matrix” printers requires a relatively constant scanning speed to ensure proper timing in the actuation of the impact pins. This is exacerbated by expected variations in machining accuracy of existing head scanning mechanisms and the operational environments in which they are used.
When the moving speed of the printing head is accelerated the striking duration of the printing pin is restricted and print darkness is deteriorated. Therefore, in order to prevent the printing darkness from being deteriorated, it is necessary to set a sufficient striking duration by retarding the speed of the printing head.
As shown in FIG. 18, the speed of a moving impact print head, as measured by the output signal of a sensor for detecting rotation of the driving motor, is fixed so that there is sufficient time for the impact pins to be turned “on” during which each impact pin extends from its rest position to make a printing impact, and also for the impact pins to be turned “off” during which each pin is retracted and returned to its rest position. However as shown in FIG. 19, if the period of the output signal of the sensor for detecting rotation of the driving motor is short (indicating that the print head is fast) there is not sufficient time for the impact pins to be retracted completely, reducing the quality of the subsequent printing impact.
Furthermore, use of existing impact print heads designed for printing on a cylindrical platen presents special problems when used to print on a platen with a different shape, such as a flat plane shaped platen found in some time recorders. FIGS. 12(a) and 12(b) show a print head having a plurality of pins “a” through “g” of a printing head B arranged in a line perpendicular to the length of the cylindrical platen A. The head shown has, by varying the angles of the printing pins, been designed to print on the curved surface of the cylindrical platen. Simultaneous operation of the pins, therefore, will produce impacts on the printed object P such as paper along the center line as shown in FIG. 12(c). The center line is referred to as the column direction and the head B scans in the row direction along the axis of the cylindrical platen A.
However, when the curved surface is replaced with one of a different shape such as the plane shaped platen D shown in FIGS. 13(a) and 13(b), simultaneous operation of the pins will instead produce the uneven “zig-zag” appearance shown in FIG. 13(c). Printing accuracy is deteriorated. Though a process of trial-and-error wherein the distance between the pins and time card C on the platen D are adjusted may improve the alignment of the impact positions somewhat, the result is generally unsatisfactory and the print quality is reduced. Redesigning the printing head to accommodate flat surfaces is undesirable because it is expensive, and the finished product will have the same disadvantage of being usable only for one type of platen.
Accordingly, there is still a need for a motor control method and an apparatus that overcomes the limitations of direct current motors in devices that feed and print cards to provide accurate speed and stop control without causing premature motor failure or transient speed variations.
There is also a need for a time recorder having a card feeder and an impact printing system incorporating the motor control apparatus that minimizes card feed time and allows printing on multiple platen shapes.